The demand for secondary batteries is increasing according to rapid development of mobile electronic devices. Particularly, batteries having high energy density are continually required to meet trends in reduction in size, weight and thickness of mobile electronic devices, and batteries, which are needed to satisfy chip, safe and environment-friendly aspects.
Lithium-sulfur batteries are secondary batteries, which use sulfur-based compounds having sulfur-sulfur combination as a positive electrode active material, and carbon-based materials, where alkali metals like lithium or metal ion like lithium ions may be inserted, as a negative electrode active material. The batteries store and produce electrical energy by using oxidation-reduction reaction, wherein the oxidation number of S is reduced by cleavage of S—S bond during reduction (when discharging), and the oxidation number is increased by reformation of the S—S bond during oxidation (when charging).
However, there is no successful example of commercialization of the lithium-sulfur battery system yet. The reason is that when using sulfur as an active material, the usage of the sulfur used in the electrochemical oxidation-reduction reaction in a battery to the added amount of the sulfur is low, and therefore, only very low battery capacity relative to the theoretical capacity is achieved.
Further, in general, sulfur atom is a non-conducting material not having electroconductivity. Therefore, an electrically conductive material, which can provide a smooth electrochemical reaction site, should be used in order for the electrochemical reaction to occur. Positive electrode structures using the sulfur atom, which have been known so far, have a structure, wherein sulfur and carbon powder as a conducting material exist independently and simply mixed in a positive electrode active material layer (mix) as described in U.S. Pat. Nos. 5,523,179 and 5,582,623. However, in this structure, the sulfur and the carbon powder as a conducting material are simply mixed. Accordingly, when charging or discharging, the sulfur is converted to polysulfide, and then flows out to an electrolyte as a liquid phase, resulting in collapse of the electrode structure. Accordingly, it has bad influence to capacity and life cycle property of a lithium-sulfur battery.
In order to solve the above problems, a method for delaying flowing out of a positive electrode active material by adding an additive, which can adsorbs sulfur, to positive electrode active material slurry is being studied. As an adsorbent for this purpose, in Japanese Patent Application Publication No. H9-147868 (Jun. 6, 1997), an active carbon fiber was used, and in U.S. Pat. No. 5,919,587, a method filling a positive electrode active material between transition metal chalcogenides, which are highly porous and have fibrous and ultra-fine sponge like structure, or encapsulating the positive electrode active material with the transition metal chalcogenides was disclosed. However, these prior arts had a problem that capacity property and life cycle property of the lithium-sulfur battery are not largely enhanced.